1. Field of the Invention
The present invention relates to the forming in integrated form of a rectifying element or rectifier.
2. Discussion of the Related Art
In the present description, an element performing a function identical to that of a bipolar diode, for example, used in D.C./D.C. converters of voltage step-down or step-up type, is designated as a rectifier.
FIG. 1 shows a conventional example of a D.C./D.C. voltage step-up converter 1 (CONV) supplying a load 2 (Q). Converter 1 provides a D.C. voltage Vout between terminals S and M of a storage capacitor 3 from a D.C. supply voltage Vin of lower level, applied between two input terminals E and M, terminal M forming, for example, a common ground with load 2. Load 2 is connected between terminals S and M of capacitor 3. For example, load 2 is formed of two light-emitting diodes of a backlighted screen.
Converter 1 comprises, in series between terminals E and S, an inductance 4 and a rectifying element 5 (RECTIF). Rectifier 5 is oriented to enable flowing of a current from inductance 4 to capacitor 3. A controllable switch 6 is connected between input terminal 7 of rectifier 5 and reference rail M. Generally, switch 6 is a MOS transistor having its control terminal formed by its gate receiving a pulse-width modulated signal PWM intended to regulate voltage Vout from either a reference voltage, or the load needs. The PWM signal, depending on the application, is provided by a control circuit not shown.
The rectifier is intended to act as a diode, that is, to block the reverse current when voltage Vout across the load becomes greater than the voltage across switch 6.
In the simplest applications, rectifier 5 is formed of a simple diode, generally of a Schottky diode. Such a Schottky diode has the disadvantage of being uneasily integrable.
Further, in many applications and especially in the case of a switched-mode application such as illustrated in FIG. 1, it is desired to be able to turn off the load supply circuit, either due to the presence of an electric problem therein (for example, a short-circuit), or for other reasons. Such a function of a rectifier element is generally designated as a true shutdown. Such a function especially enables, if switch 6 is off, preventing supply voltage Vin present between terminals E and M from recharging capacitor 3 while this is not desired. This especially enables performing a protection function by preventing overcurrents in case of a short-circuit on the load side (between terminals S and M).
To perform this function, a rectifier 5 formed of a diode D in series with a switch 8 is thus generally used, as illustrated in FIG. 1. Switch 8 is controlled by a circuit 9 (CTRL) receiving an enable signal EN from a control organ of the converter. Enable signal EN is used, for example, to interrupt the converter operation by the turning-off of switch 8 in case of a short-circuit in the load (detected by means not shown), in case of a problem on the side of the switched-mode power supply (PWM) control circuit, or more generally as soon as a turning-off of the converter is desired to be guaranteed. In practice, switch 8 is generally formed of a MOS transistor.
A disadvantage of rectifier 5 of FIG. 1 is the addition, in normal operation, of a voltage drop due to the on-state series resistance of switch 8 (Ron of the MOS transistor).
Another disadvantage is linked to the bulk of the high-voltage MOS transistor forming switch 8.
It has also been provided to form the rectifier in the form of a single MOS transistor by using its parasitic bulk-source diode. Such a solution requires being able to control the bulk connection to ground to control the parasitic diode. When a circuit shutdown is desired, the polarities of the MOS transistor and of its bulk are reversed to reverse the polarity of its parasitic diode. Another disadvantage is that the actual transistor must be controlled at the frequency of the PWM signal. In normal operation, the transistor is controlled in reverse fashion with respect to switch 6.
In addition to the difficulty linked to the need for controlling the transistor at the frequency of the PWM signal, the source of the transistor is floating, which poses problems of control reference of its gate signal. This generally results in using level-shifting circuits, which makes the circuit particularly complex, especially for a high-voltage operation.
The series association of diode D with switch 8 could have been devised to be replaced by a thyristor. However, such a solution is incompatible with switched-mode power supplies. Indeed, a thyristor would require being controlled to be turned off each time switch 6 is off. Now, the frequencies of the trains of width-modulated pulses are generally of several tens, or even hundreds of kilohertz, which is incompatible with switching rates currently available for thyristors.